I tried doing that the other night. It was raining; I found the wet grass sucked the heat out of me through the bivvy. The material is thin and offers no insulation against conduction. I quickly moved under some trees, where there was a bed of pine needles. This was much warmer (and dryer).

The twigs poked a dozen or so holes in the material, but that didn't seem to matter much at the time. The holes didn't grow. I've now put duct tape over the worst of them and left the others. I think the bivvy is still usable but I've swapped it out of my everyday carry.

I spent about 90 minutes on the test. I was wearing thin office trousers, and wasn't really comfortable enough to spend the night by choice.

I was impressed by their small size when carried and large size in use. In use they are about 7'x3', which is easily enough to lie full length with your head covered. Much better than any ordinary plastic bag I've found. When carried they are small enough to fit in a tin mug with room over. I found I could re-pack them easily (indoors) - any trapped air somehow escapes and it's no trouble putting them back into their bag. I have chosen to fold mine rather than roll it, to make a flatter form-factor which will fit better into my EDC waist-pack.

It's best to have a pee before you get into the bivvy. Don't get in, lie there for an hour and then think to yourself, "I'd really like to have a pee now."

Overall I'd strongly recommend them, so long as you are realistic about their limitations.